Integrating sphere photovoltaic receiver (powersphere) for laser light to electric power conversion

ABSTRACT

A photovoltaic module for converting laser radiation from a laser emitting light at a wavelength to electrical power is provided. The module comprises: (a) a housing having a cavity of generally optimized closed shape inside the housing, the cavity having an internal surface area A s  and including an opening for admitting the laser radiation into the cavity, the opening having an entrance aperture area A i  that is substantially smaller than A s ; and (b) a plurality of photovoltaic cells within the cavity, the photovoltaic cells having an energy bandgap to respond to the wavelength and generate the electrical power.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part application of Ser.No. 10/151,640, filed May 17, 2002, and recently allowed, the contentsof which are incorporated herein by reference.

TECHNICAL FIELD

The present invention is related generally to laser power beaming,employing photovoltaic cells, and, more particularly, to a novelphotovoltaic module for converting a directed laser beam into electricalpower.

BACKGROUND ART

State-of-the-art single junction solar arrays as well as concentratorsusing single junction solar cells utilize only a limited portion of theavailable solar spectrum, thereby wasting the remainder of availableenergy outside of their limited spectral response. The limitation iscaused mainly by two basic “photon loss” mechanisms within the cells,namely, (1) loss by longer wavelengths and (2) loss by excess energy ofphotons. In the former case, photons with energy smaller than the“energy bandgap” or “forbidden gap” E_(g) (direct bandgap semiconductor)or E_(g)−E_(Phonon) (indirect bandgap semiconductors where E_(phonon) isthe phonon quantum of energy) cannot contribute to the creation ofelectron-hole pairs. In the latter case, in the spectrum range ofinterest, one photon generates only one electron-hole pair. The rest ofthe energy larger than the bandgap is dissipated as heat. Photons withenergy hv≧E_(g) thus can only use a portion of E_(g) of their energy forgeneration of electron-hole pairs. The excess energy raises thetemperature of the solar cell and degrades its performance. Thus, evenhigh quality cells with excellent quantum efficiencies, such as GaAs,exhibit relatively modest conversion efficiencies since they cannotrespond to more than a relatively small portion of the incidentspectrum.

One way of circumventing this limitation is the use of two or moredifferent bandgap cells that are stacked, or monolithically grown, in avertical manner. Such a multi-junction (MJ) system with appropriatelychosen bandgaps can span a significantly greater portion of the incidentsolar spectrum than achievable with single-junction cell systems. Suchmulti-junction solar cells are well-known. For example, three-junctioncells have been devised that can control a relatively larger portion ofthe solar spectrum, and are further described below. Because of theirpotential for very high efficiencies, MJ cells have enjoyed increasedinterest over the last two decades.

At a NCPV (National Center for Photovoltaics) meeting in Denver, Colo.on Apr. 16-19, 2000, it was reported that triple-junction GaInP2/GaAs/Geconcentrator cells developed by NREL (National Research EnergyLaboratory) and Spectrolab have achieved 32.3% at 47 suns and 29% at 300suns (AM1.5, 25C), with an obvious drop of 3.3% (absolute) or 10.2%(relative), indicating one of the many limitations of MJ concentratorsystems at higher concentrations. It should be kept in mind that theabove-mentioned encouraging achievement with a pulsed solar simulatordoes not represent a real life situation. Under actual operatingconditions, the MJ concentrator system performance can drop more than 12to 15% (absolute) against the bare cell performance and defeat the useof high efficiency MJ cells. Some of the major concentration-relatedperformance losses in MJ cells are caused by the following shortcomings:absorption of light in the top cells, chromatic aberrations caused bythe concentrator optics, flux non-uniformity on the cells, limited heatremoval from the top cells, current limitation in the cells, seriesresistance, shadowing losses due to finger contacts on the cells, andlimited acceptance angle for photon incidence on the cells. Most ofthese limiting factors apply to all conventional concentrator typesbased on a variety of cells. MJ cells, however, are more vulnerable tomost of these performance-limiting factors.

The relative deterioration of MJ cells becomes worse as the number ofjunctions increases. Several authors in the field have predicted thatfor vertically stacked or monolithically-grown systems, limitedimprovements are expected beyond triple-junction cells. A recent pressrelease by Boeing (Spectrolab) on Aug. 15, 2001, confirmed that a triplejunction cell developed by Spectrolab and NREL has reached a conversionefficiency of 34% (a world record at that time) at 400×. That appears tobe very much the limit of three-junction cells. Four-junction cells arepredicted to be able to reach upper 30% and lower 40% efficiencies.Theoretical studies have shown that to achieve this kind of efficiencylevel, a four-junction cell system requires a 1 eV bandgap III-V cellthat meets all requirements including: optical, thermal, and electronicissues involved. In spite of extensive efforts, this material remainselusive.

Another shortcoming of the monolithic MJ cells lies in the limitation ofcomplementary bandgap cell materials with matching lattices. Invertically-grown MJ cells, all the adjacent “sub-cells” must havematching or slightly mis-matching lattices for proper performance. Thus,even the best bandgap matched sub-cell cannot result in a multi-junctioncell if their lattices mis-match. This requirement narrows downsignificantly the available set of sub-cells that could be used.

These apparent limitations represent a formidable bottleneck in thedevelopment of high and very high efficiency (and thereforecost-competitive) concentrator systems in the near future. According toanalytical studies, ideal four bandgap cell systems utilizing a new 1 eVmaterial can improve the solar to electricity conversion efficiency over48% at 500 suns. Even at a cost of $250/Watt for such a system, theeffective cell system cost for a 500× flux concentrator can be as low as$0.50/Watt. At this cost level, the concentrators would be ahead of thelong range goals of the Department of Energy for PV flat platetechnology (installed system cost of $1.00/Watt to $1.50/Watt by theyear 2030), if the balance of concentrator system could be built for$0.50/m² to $1.00/m². Thus, very high cell and system efficiencies areparamount to achieve the long term cost goals for photovoltaics ingeneral.

In the late 1990s, NASA and JPL scientists proposed an alternativetechnique, called “Rainbow”, to circumvent the problems of vertical MJsystems and improve the performance of multi bandgap cell systems. Theirmethod is to split the solar spectrum into several frequency bands andfocus each frequency band onto separate cells with corresponding energybandgaps. The Rainbow multi-bandgap system represents a combination ofsolar cells, concentrators, and beam splitters. The use of separatediscrete cells offers the widest possible scope of semiconductorchoices. Based on data for “real” cells and optical components, Rainbowwas expected in 1997 to convert over 40% of incident solar energy toelectricity at the system level.

To the knowledge of the present inventor, this concept has never come toa closure, presumably due to extreme difficulties encountered with theassociated optics. In addition, this space system would only have aconcentration ratio of a maximum of 20×, i.e., much lower than the 500×or more to reduce the effective cell cost dramatically. A thoroughliterature search has shown that in the past, the very promising methodof spectral splitting and simultaneous use of discrete solar cells withdifferent bandgaps has never reached its potential capacity and thetechnology was never exploited fully. The parent application to thepresent application represents a straight-forward approach to achievebreak-through performance levels and with it to rapidly lower the costof solar energy to competitive levels.

To address the large demand for noise-free and safe power transmission,without the use of electrical wiring, several new technologies are beingintroduced. The two major approaches are: (1) microwave and millimeterwave beaming and (2) optical fiber light transmission in conjunctionwith optically powered, sensors, transducers and data communicationsequipment. At the receiving end, microwaves and millimeter waves areconverted into electricity via highly tuned phased array antennas. Inthe case of optical fiber power transmission, the conversion of lightinto electricity happens via a photovoltaic power converter, which isbasically a slightly modified solar cell.

The conversion of beamed microwaves and millimeter waves into electricpower is highly efficient. However, concerns with the potentialhazardous impact of high intensity beams and the strong beam divergencelimit the area of applicability of such power-beaming technologies tohigh altitudes and space. Optical fiber power transmission is distance-and power-limited due to optical absorption in the fiber and lightinput/output coupling losses. Most of the reported fiber optics powertransfer applications are limited to local area networks (<<1 km) ofpower levels less than 1 watt and for the most a few microwatts. Thus,there is a need for a power beaming technology that can provide awireless electric power source ranging from 1 watt to tens of kWatts andcan be beamed from, say, 10 meters to several kilometers and beyond.Such high laser power levels are now available, due to emerging lasertechnologies such as chemical oxygen-iodine lasers (COIL) that arescalable up to 40 kW at a wavelength of 1.315 microns.

More recently, proposals have been made to convert coherent light toelectricity. Such applications have been termed “Laser Power Beaming”(LPB). LPB technology uses the properties of coherent light to transferpower between two locations without the need of any material or man-mademedium. Thus, LPB is extremely fast and weightless. Over the lastdecade, total energy efficiencies for some lasers have improvedsignificantly (40% and up) and reliable operation of high power lasersover long periods of time has been demonstrated in real lifeapplications. The most efficient method of converting beamed laser powerinto electricity at the receiving end is the use of photovoltaic (PV)cells. As a result of recent research and development efforts on solarPV cell technology, solar-to-electricity conversion efficiencies as highas 36% has been achieved at 500×AM1.5 suns or about 50 W/cm².Efficiencies for monochromatic light, as it is the case with LPB, areexpected to be much higher. Research efforts in the field ofthermo-photovoltaics (TPV) made it possible to develop new photovoltaicmaterials that are responsive in the near infrared range of theelectromagnetic spectrum, that would, for example, operate at 40 to 45%efficiency at 1.315 wave length of the COIL lasers mentioned above. Sucha TPV cell, for example, GaInAsSb/AlGaAs, can be used effectively withthe COIL lasers mentioned above.

As an aside, it is important to note that in the past, integratingsphere systems have been used to measure, control, and monitor laser andlaser diodes. However, to the knowledge of the inventor, the PowerSphereapproach disclosed and claimed herein is the first disclosure thatteaches how the integrating sphere concept can be exploited to convertbeamed laser energy into electric power.

DISCLOSURE OF INVENTION

In accordance with the present invention, a photovoltaic module, orPowerSphere, for converting coherent laser radiation from a laseremitting light at a wavelength into electrical power is provided. Themodule comprises:

-   -   (a) a housing having a cavity of generally optimized closed        shape inside the housing, the cavity having an internal surface        area A_(s) and including an opening for admitting the laser        radiation into the cavity, the opening having an entrance        aperture area A_(i) that is substantially smaller than A_(s);        and    -   (b) a plurality of photovoltaic cells within the cavity, the        photovoltaic cells having a bandgap energy to respond to the        wavelength and generate electrical power.

Further in accordance with the present invention, a combination of areflecting concentrator and the photovoltaic module is provided. Thereflecting concentrator comprises:

-   -   (a) a primary concentrator for intercepting and concentrating        the laser radiation, and    -   (b) a secondary concentrator for receiving the concentrating        said laser radiation from the primary concentrator and further        concentrating the laser radiation.

The photovoltaic module is positioned for receiving the furtherconcentrated laser radiation from the secondary concentrator.

The PowerSphere of the present invention has the potential to yieldlaser-to-electricity conversion efficiencies from 60% to 70%.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand accompanying drawings, in which like reference designationsrepresent like features throughout the FIGURES.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referred to in this description should be understood as notbeing drawn to scale except if specifically noted.

FIG. 1 is a cross-section view, in schematic, depicting the basicprinciples of the multi-bandgap Photovoltaic Cavity Converter (PVCC);

FIG. 2 is a view similar to that of FIG. 1, depicting the escapeprobability of a photon representing a discrete frequency band from thePVCC;

FIG. 3 is a cross-section view, similar to that of FIG. 1, depicting theprinciples of the single bandgap integrating sphere photovoltaicreceiver (Power-Sphere) operation with a dielectric light injector;

FIG. 3 a is an enlargement of a portion of FIG. 3;

FIG. 4 is a schematic diagram, depicting a Cassegranian concentrator,coupled to the PowerSphere, to accommodate beam broadening for longrange power beaming applications for both space and terrestrial use;

FIG. 5 is a schematic diagram, depicting a deployed space vehicleequipped with a combination of laser and solar power modules;

FIG. 6 is a schematic diagram, depicting a sequence of a continuousflight drone powered by laser beaming; and

FIG. 7 is a schematic diagram, depicting the use of the PowerSphere as asource of DC power, free of electromagnetic interference (EMI).

BEST MODES FOR CARRYING OUT THE INVENTION

Reference is now made in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventor for practicing the invention. Alternative embodiments are alsobriefly described as applicable.

The embodiments herein are directed to an integrating spherephotovoltaic (PV) receiver, or module, for converting laser light intoelectric power. The receiver is called a “PowerSphere”.

The basic concept of the PowerSphere is based on a photovoltaic cavityconverter (PVCC) module that has been designed for a concentration inthe range of 500 to over 1000 suns and a power output range of a fewkilowatts to 50 kW_(e) when combined with a primary dish and a secondaryconcentrator. That PVCC module is disclosed and claimed in theabove-identified parent application, now U.S. Pat. No. ______. The PVCCmodule herein is expected to find use in, for example, DOE'sConcentrating Solar Power (CSP) program to develop systems in the 1 to 5kW_(e) and 10 to 30 kW_(e) size ranges based on reflective optics. Atypical power range is about 30 to 50 kiloWatts. Connecting a pluralityof such modules together in a power plant permits power generation up toseveral hundred mega Watts.

The PVCC module is based on advanced single junction cells, includingIII-V cells, for example, manufactured by EMCORE Photovoltaic(Albuquerque, N.M.). The PVCC module is based on reflective optics, andis capable of delivering power in the range of 0.5 to 3 kW_(e) atconcentrations in the range of 100 to 500× when optically coupled to theexit aperture of the second reflective stage (CPC) currently located atHFSF. According to NREL specifications, this second reflective stageprovides an average flux density of 20,000 AM1.5 suns at its exitaperture. The overall targeted module conversion efficiency for near-and midterm is to exceed 33% to 45%, respectively.

The PVCC module is a light-trapping cavity equipped with internal solarcells of different energy bandgaps. A unique system of Rugate filters isapplied to the cells to “split” the solar spectrum by the method ofselective energy extraction (spectral screening). This novel conversiondevice actually defocuses to a certain extent the pre-focused solar fluxentering the cavity in a controllable manner by determining the diameterof the sphere.

FIG. 1 illustrates the principles of the Photovoltaic Cavity Converter(PVCC). In FIG. 1, the PVCC 10 comprises a housing 12 having an internalcavity 14 that is generally spherical, but may be some other optimizedclosed shape. By an “optimized closed shape” is meant, for example, agenerally spherical shape, a generally ellipsoidal shape, or a generallyconical shape. In general, any shape that is closed upon itself isuseful. However, a generally spherical shape is preferably employed. Theclosed shape is optimized to promote an efficiency that is as high as ispossible (“optimized”) in the collection of photons.

The cavity 14 contains therein a plurality of solar cells 16, groupedinto voltage-matched cell strings of different energy bandgaps.Simultaneous spectral splitting occurs by means of selectivetransmission and/or reflection of the photons by matching (conjugated)Rugate filters 17 associated with the cells 16. Alternatively, acombination of Rugate filters and stack interference filters may be usedas filters 17. In an exemplary embodiment, there are four groups ofsolar cells, denoted 16 a, 16 b, 16 c, 16 d, although it will beappreciated by those skilled in this art that less than four groups ormore than four groups of solar cells may be employed. In a preferredembodiment for PVCC, four or more cell types are employed, which, whenproperly selected, is expected to result in higher efficiencies.

Each group of solar cells 16 is responsive to a different portion of thesolar spectrum 18. Examples of suitable solar cells that are responsiveto different portions of the solar spectrum are discussed below.

The light 18 entering into the spherical cavity 14 is first pre-focusedby a primary concentrator (dish) (not shown in FIG. 1) and then by asecond-stage, or secondary, concentrator 20 that has its inner surfaces22 mirrored. An example of such a second-stage concentrator is disclosedin U.S. Pat. No. 6,057,505, issued May 2, 2000, to the present inventor.The second-stage concentrator 20 has a Bezier optimized contour toprovide a combination of maximum acceptance angle, highestconcentration, and minimum height.

After passing through the second-stage concentrator 20, the light 18then enters the spherical cavity 14 through a small entrance aperture 24(similar to an integrating sphere) and is defocused to the desired fluxconcentration by the choice of the diameter of the sphere 12. The escapeprobability of the trapped photons can easily be kept below a fewpercent by making the aperture 24 small enough as compared to thesurface area of the interior wall 26. The highly reflective interiorsurface 26 of the sphere 12 is lined with discrete single junctioncells, including III-V solar cells 16 of different energy bandgapsand/or IV solar cells, such as Si and/or Ge. Other types of cells arealso permissible if they meet the performance criteria.

Photons, once trapped by the cavity 14, undergo several bounces from thecells 16 and cavity wall 26 until they are either (1) absorbed togenerate waste heat or (2) transmitted into the appropriate cells togenerate electron-hole pairs with a high probability or (3) escape backto space through the aperture 24. The probability of escaping throughthe aperture 24 is dependent to a first approximation upon the ratioA_(i)/A_(s), where A_(i) is the diameter of the aperture and A_(s) isthe diameter of the sphere 12. A small A_(i) and a large A_(s) means asmall escape probability. Preferably, the ratio of A_(i):A_(s) is lessthan 0.01.

As shown in FIG. 1, beam 18 is depicted as comprising photons at fourdifferent wavelengths λ₁, λ₂, λ₃, and λ₄. Each of the solar cells 16 a,16 b, 16 c, and 16 d are each responsive to a different wavelength. Inthis example, λ₁ is associated with solar cell 16 a, λ₂ is associatedwith solar cell 16 b, etc.

For example, diffusely-reflected λ₄ photons, denoted at 118′, arereflected from the cavity wall 26. A λ₃ photon, denoted 118 c, enters amatching Δλ₃ solar cell 16 c. As another example, λ₁ photon, denoted 118a, is rejected by a Δλ₂ solar cell 16 b, but is absorbed by solar cell16 a.

A. Photon Capture by the Spherical Cavity (Photon Escape Probability)

The highly concentrated beam (photons 18 in FIG. 1) from the secondaryconcentrator 20 is injected into the spherical cavity 14 and is trappedwithin the boundaries of the cavity wall 26. (In actuality, the beam 18becomes divergent or defocuses after entering the cavity 14 at aperture24.) The escape probability of a trapped photon representing a frequencyband is, to a first degree, proportional to the ratio of entranceaperture area (A_(i)) to the total interior surface area (A_(s)) of thesphere. FIG. 2 illustrates the escape probability of a photonrepresenting a discrete frequency band.

The incoming flux of photons is represented by 218 a and the outgoingflux by 218 b. The entrance aperture 24 has area A_(i). The photon 218 acan enter a solar cell 16 a or be reflected off its surface and enteranother solar cell 16 b, or the photon can be reflected off the surfaceof the first solar cell 16 a and in turn reflected off the interiorsurface 26 or reflected back through the entrance aperture 24. A Rugatefilter 17, for example, 17 a, is shown associated with each solar cell16, for example, 16 a. The Rugate filter 17 may be formed directly ontop of the solar cells 16 or deposited on a fused glass cover and may becemented to the cell or spaced apart from the solar cells.

Using the integrating sphere radiance equation, it can be shown that fora given frequency band the escape probability for a photon within thatband is given to a first order by:

Q _(out) /Q _(in) =A _(i) /A _(s){r(1−f)/1−(1−f)},

where Q_(out)/Q_(in) is the ratio of the outgoing flux 118 b to theincoming flux 118 a, f=(A_(i)+A_(e))/A_(s), A_(s)=total interior surfacearea of the sphere 14, A_(i)=area of the entrance aperture 24, andA_(e)=total surface area of the cells 16 in the given frequency band,for example, cell 16 a. This simplified equation assumes that the cells16 (i.e., cells 16 a, 16 b, 16 c, 16 d) with bandgaps outside thefrequency band have the same reflectance as the highly reflective spherecoating 26 a (discussed in greater detail below). Now consider a sphere14 of 10 cm diameter with an entrance aperture of 1.4 cm in diameter.The interior of the sphere is 20% occupied with cells of a givenbandgap. All cells 16 are coated with Rugate filters 17 with complete(100%) transmission and reflection characteristics, respectively. Thecalculated escape probability becomes then about 1.2%. This probabilitycan be reduced by choosing a larger diameter for the sphere 14. This,however, will lower the overall concentration ratio (see below) andincrease the absorption of the photons by the uncovered cavity wall.

B. The Choice of Diffuse Reflector

The quality of the diffuse reflector 26 a on the wall 26 of the cavity14, shown in FIG. 1, is important. It is directly related to reflectionlosses, flux uniformity, and sphere multiplier factor M. The stabilityof the reflector under high levels of flux and possible, accidentaltemperature runaways is also very important as these may change thereflectance.

Preliminary evaluation of reflector materials 26 a resulted in theselection of “space-grade” SPECTRALON that combines veryhigh-reflectance with an extremely lambertian reflectance profile. Italso has excellent low outgassing features. SPECTRALON, which ismanufactured by Labsphere (North Sutton, N.H.), is a thermoplastic resinwith special pigments added that can be machined into a wide variety ofshapes for the construction of optical components. The material ischemically inert and is thermally stable up to 400° C. Further detailsof this material are disclosed and claimed in U.S. Pat. No. 5,763,519,entitled “Diffusively Reflecting Sintered Fluorinated Long-ChainAddition Polymers Doped with Pigments for Color Standard Use”, andissued to A. W. Springsteen on Jun. 9, 1998, incorporated herein byreference.

SPECTRALON reflectance material gives the highest diffuse reflectance ofany known material or coating over UV-VIS-NIR (ultraviolet-visible-nearinfrared) region of the spectrum. The reflectance is generally >99% overa range from 400 to 1500 nm and >95% from 250 to 2500 nm. The materialis also highly lambertian at wavelengths from 250 to 10,600 nm. A“lambertian” surface is a perfectly diffusing surface having theproperty that the intensity of light emanating in a given direction isproportional to the cosine of the angle of the normal to the surface(lambertian cosine law). A material such as SPECTRALON obeying this lawis said to be an isotropic diffuser that has the same radiance in alldirections. “Highest diffuse reflectance” means the known highestreflectance of 99.1% over the solar spectrum. Another suitablereflectance material for use as the coating 26 a in the practice of thepresent invention is barium sulfate.

C. Integrating Sphere Photovoltaic Receiver

In accordance with the present invention, since the integrating spherePV receiver, or PowerSphere, is intended for a single wavelength lasersource, then only one type of solar cell, sensitive to that wavelength,need be used. For the same reason, Rugate, or other spectral filters,are unnecessary.

The laser-to-electricity conversion approach, or PowerSphere, isdepicted in FIG. 3. As with the PVCC 10, the PowerSphere 310 includes anoptimized closed shape, preferably, a spherical cavity 314 with a smallport 324 for insertion of light 318 and an array of photovoltaic cells316 that almost completely cover the interior wall 326 of the cavity. Asecondary, dielectric concentrator, or light injector/booster, 320 withan extractor rod 336 serves the same function (i.e., boosting andinjecting pre-focused laser beam into the sphere 310) as the secondaryconcentrator 20 discussed above. However, the extractor rod 336 evenlydistributes the light in all directions inside the cavity 314. As withthe secondary concentrator 20, the inner surfaces 322 of the secondaryconcentrator 320 may be mirrored. Preferably, the secondary concentrator320 is a non-imaging, compound parabolic of hollow design, as disclosedin the above-referenced U.S. Pat. No. 6,057,505.

The individual cells 316 are interconnected with each other in a certainfashion (in parallel and in series) to achieve the voltage and currentlevels for the required electrical power output. The laser light 318enters the cavity 314 via the dielectric concentrator 320. As shown inFIG. 3, the E-rod 336 fits exactly the light entry port 324 of thesphere 310. The function of the E-rod 336 is first to guide the light318 towards the center of the cavity 314 and then to emit the photons318 uniformly in all directions. This angular isotropy is required toachieve the best possible flux uniformity before the photons 318experience their first reflection at the cavity wall 326. Strongnon-uniformities in the flux distribution have a degrading effect on theperformance of the array of the photovoltaic cells 316.

Dielectric, non-imaging, secondary concentrators based on total internalreflection are abbreviated as DTIR. This type of refractory secondarywas introduced in the mid-1970s to enhance the performance ofreflective, non-imaging, two-stage concentrators for larger acceptanceangle at a given concentration ratio and to reduce the focal distance ofthe two-stage concentrator system. In addition, they proved to havehigher throughput than refractive systems.

An E-rod 336 works by multiple reflections of light rays at increasinglyincreasing angles along its length, causing each ray to eventually failTIR and refract out of the E-rod into a lower index medium. In the caseof the PowerSphere 310, the low index medium is air and the isotropicdistribution of the rays are achieved by facets in a certain pattern onthe surface of the E-rod 336. The impact of the use of the E-rod 336 isthe elimination of the lambertian material 26 a (e.g., SPECTRALON). Inthe PowerSphere design, the photons 318 are distributed evenly and thephotovoltaic cells 316 see a uniform flux to start with.

The photovoltaic cells 316 are carefully selected with regard to theirquantum efficiency to optimize the conversion of the monochromatic light318 of the chosen laser. The best conversion is achieved when the laserwavelength coincides with the peak of the quantum efficiency response.For laser power beaming applications, the cell design must take intoconsideration the very high flux concentrations that are involved. Therequired cell design features for laser power beaming are achieved byproper doping of the cells 316 and by enhancing the top metal contacts(grid fingers) (not shown) of the cell in order to mitigate the rapidlygrowing series resistance as the flux density increases. As is explainedbelow, the PowerSphere concept allows much wider and denser grid fingersthan the conventional flat plate PV receivers without the associated“shadowing” losses. Further, an anti-reflective coating 316 a may beformed on the front surface of the photovoltaic cells 316, as shown inFIG. 3 a. Thus, in principle, the PowerSphere can reach higherefficiencies than possible with the flat plate PV receivers at high fluxdensities.

The photovoltaic cells 316 may be provided with a thin mirrored backsurface 316 b for reflecting photons not absorbed by the bulk of thecell. Such reflected photons, however, are absorbed by another PV cell316 inside the cavity 314. Thus, the photon utilization factor isimproved.

The PowerSphere is a novel technology that aims at the highestlaser-to-electric power conversion efficiency for power beamingapplications. Modified versions of the PowerSphere concept areapplicable to both space and terrestrial applications. For example,NASA, the Air Force, and others are intensively exploring utilization oflaser power beaming in space. All these applications requirephotovoltaic laser beam receivers that are highly efficient andreliable. FIG. 4 shows the principles of a lightweightCassegranian/PowerSphere for space applications, although such aconfiguration could also be suitably employed, with minor modifications,for terrestrial applications.

Specifically, as shown in FIG. 4, a laser power beam 318 is interceptedby a reflecting concentrator 400, e.g., a Cassegranian concentrator,comprising a primary concentrator 434 that is preferably parabolic inshape and also serves as a heat radiator. A secondary concentrator 434′that is preferably hyperbolic in shape is located at the focus of theprimary concentrator 434, and directs the laser light 318 into the lightinjector/booster 320, which can be either refractive or reflective. Thelight injector/booster 320 serves essentially the same function as thesecondary concentrator 20, as described above. The secondaryconcentrator 434′ is suspended above the primary concentrator 434 bytelescopic arm 456. The light 318 then enters the integrating spherephotovoltaic receiver 310 through the injector/booster 320, as describedabove.

A loop heat pipe system designed for zero gravity space environment,shown at 450 and described in greater detail in above-referenced U.S.Pat. No. ______ [D-2K042], removes waste heat from the sphere 310 andtransfers it to the back surface 434 a of the primary concentrator 434.The rejected heat 452 is radiated into space (or the surroundingenvironment) by the primary concentrator 434. The primary concentrator434, a parabolic dish, that intercepts the somewhat broadened laserbeam, is made of highly conductive carbon fiber composite for excellentsurface thermal diffusivity, stiffness and lightweight. A non-limitingexample of such a highly conductive carbon fiber is K1100, available,for example, from AMOCO. The back-surface 434 a of the primaryconcentrator 434 is highly emissive to facilitate good radiative wasteheat rejection into space or surrounding environment.

FIG. 5 illustrates how a space vehicle 500 can be equipped with both alaser PV module 310 and a solar PV module 10 if the missions powerdemand requires two independent power sources. The two modules 10, 310,are deployed from the space vehicle 500 by deployment booms 502. Atleast the laser PV module 310 has rotational capability about axes A andB to optimize interception of the laser power beam 318. In this latterconnection, the back surface of the secondary concentrator 434′ (facingthe laser beam 318 or sun's rays 18) may be provided with centeringmeans (not shown) to align the concentrator 434 to incoming radiation18, 318. An example of one such means is disclosed in U.S. Pat. No.4,330,204, entitled “Self-Aligning Laser Communicator UtilizingReciprocal Tracking” and issued to Richard A. Dye on May 18, 1982, thecontents of which are incorporated herein by reference. In essence, aquadrature detector, composed of four equal segments, receivesradiation. So long as all four segments generate the same current, nofurther alignment is necessary. An imbalance in current is indicative ofmis-alignment (off-center position) of the beam, and the information canbe used to rotate the PV module 310 about axes A and/or B to bring theconcentrator 434 into alignment.

It is also possible to construct a “dual purpose” PVCC that can convertboth direct solar radiation and a directed laser beam efficiently intoelectricity. Such a PVCC would find application in certain spacemissions where the satellite must fly through an eclipse during which nosolar radiation is available. However, a laser based on a space platformat a suitable distance and position can provide the power during theeclipse period. Such a dual purpose PVCC may contain a combination ofmulti-junction cells and single junction cells, such as InGaP/GaAs(multi-junction), silicon (single junction), and InGaAsP/InGaAs(multi-junction). The dual purpose PVCC would essentially combine thesolar cells 16 a-16 d with filters 17 of FIG. 1 and the photovoltaiccells 316 of FIG. 3 in a single cavity 14 or 314. The solar cells 16a-16 d would be selected to span at least a portion of the solarspectrum.

There is also a growing need for terrestrial power beaming with lasersfor a multitude of applications where electric power cables cannot beused or are not practical and also the use of batteries is limited.Examples include: energy transfer to rotating systems or flying unmanneddrones (see FIG. 6), potentially explosive surroundings, facilities forradioactive and other hazardous materials, remote robotics, power supplyto switches, remote sensor applications with high power demand, highpower electronic systems (e.g. telemetry equipment as shown in FIG. 7)susceptible to electromagnetic interference (EMI), unmanned surfacemaritime vessels, off-shore oil exploration, etc.

In FIG. 6, an unmanned drone 600, such as used for border surveillance,is shown moving across the page. The drone 600 is fitted with anintegrated sphere photovoltaic receiver 310, such as described above. Aground-based laser network, comprising a plurality of lasers 602 a, 602b, 602 c emitting laser radiation 318 that is receivable by the PVreceiver 310, is established over an area to be patrolled by the drone600. For improved reception of the laser power beam 318, the PV receiver310 could be mounted on gimbals (not shown), which would permit thedrone 600 to track the laser power beam 318 as the drone moves from afirst laser 602 a to a second laser 602 b to a third laser 602 c. Itwill be appreciated that there is a beam transfer range 606 as the drone“passes off” from one laser, e.g., 602 a, to another laser, e.g., 602 b.

For an unmanned drone 600 flying at an altitude of 35,000 feet, thedistances between each two beam transfer range (604 a, 604 b, 604 c)would be about 13.4 miles, if the traveling power beam 318 is allowed tosweep an angular distance from −45° to +45° around a point (zenith)directly overhead on a given laser beam source 602 in the laser networkthat covers a specified area. In this configuration, the distancesbetween neighboring laser sources 602 would be also about 13.4 miles.

In FIG. 7, a missile 700 is equipped with the PowerSphere 310 of thepresent invention, which provides DC power along line 702. ThePowerSphere 310 receives light 318 from a high power laser 704. The DCpower provides a source of power to telemetry equipment 706 in themissile 700, where, for example, the equipment is undergoing longduration ground testing and it is required that the power be free ofelectro-magnetic interference.

High power lasers have been under intense investigation as a directedenergy source. The U.S. Government and industry have a long standinginterest in developing high-power lasers for a variety of applicationsincluding materials processing, isotope separation, nuclear fusion, longrange sensing and long range communications and other defenseactivities. Key development goals are high brightness and highefficiency. Laser technologies that are in an advanced stage ofdevelopment include chemical oxygen-iodine laser (COIL), photolithiciodine lasers, carbon dioxide lasers, diode pumped solid-state lasers,and high power semiconductor diode lasers. High power semiconductordiode Laser technology has some definite advantages over other types oflasers because of their extremely small size and efficiency (potentiallyup to 70%). Chemical oxygen-iodine lasers have also been proven to bescalable up to 40 KW with improved efficiency.

In the past, several semiconductor cell materials have been studied byNASA in conjunction with PV converters for laser power beaming. The mostrigorously studied PV materials are Si, GaAs, InP, Ge, and certain III-Vcells, including InGaP, InGaAs, and InGaAsP. All these cells havereached a mature technology state and are commercially available.Silicon cells, although not the most efficient of them all, are the bestinitial choice because of their reliability, availability, and low cost.

Over the last decade “bandgap engineering” studies have opened up thepossibilities to a new era of bandgap “tunable” semiconductors. Someexamples are recently discovered indium/gallium alloys and quantum-wellsystems. Once these technologies are fully developed, it will becomepossible to closely match a given laser frequency with the quantumefficiency peak of a tunable PV cell. This is particularly important forthe near-infrared range where COIL lasers with good atmosphericpenetration can be matched with the recently developed photovoltaicGaAlInAsSb alloys with bandgaps ranging from 0.52 to 0.55 eV.

D. Expected Laser-to-Electric Power Efficiencies with PowerSphere:

Some fiber optic driven photovoltaic power converters are already on themarket. These devices are quite efficient (about 40%) but their powercapability is very low (about a few mW). The Power/Sphere systemdisclosed herein, however, involves a high power laser/PV converter thatcan generate electric power ranging from few watts to tens of kilowatts.As mentioned above, such high power concepts have been mostly ofinterest to NASA and DOD, and the bulk of the available data comes fromthese sources.

Theoretical modeling by NASA indicates that by tuning the wavelength ofa laser to 840 nm, a power beam system based on GaAs can achieve quantumconversion efficiencies approaching 60%. The highest literature cellefficiency reported under selective illumination is 59% for theAlGaAs/GaAs hetero-junction cell, at laser input intensities up to 54W/cm². The integrating sphere receiver of the present invention isexpected ultimately to reach efficiencies approaching 70%. Thisimprovement is due to minimized series resistance at very high fluxdensities and the photon recycling process in the cavity, as explainedbelow.

E. PowerSphere vs. Flat Plate PV Receivers:

There are fundamental differences between the operational principles ofPowerSphere 310 and flat plate PV receivers for concentrator systems.These are briefly highlighted below:

-   -   1. Photon utilization:

Incident photons (laser beam) on a flat plate PV receiver either enterthe solar cells or are reflected from the active cell surface and fromthe top surface metallization (grid fingers, bus-bar, etc.). Photonsstriking the non-active areas between the cells are either absorbed orreflected. Reflected and absorbed photons are lost for the conversionprocess and can no longer contribute to the photocurrent. These lossesare substantial in the case of high flux densities such as the laserpower beaming require (50 Watts/cm² or higher).

In contrast to flat plate receivers, the PowerSphere 310 shown in FIG. 3traps almost 99% of the photons 318 that enter the cavity 314. Reflectedphotons 318 b return back into the cavity 314 and are recycled. A highreflectivity material (not shown), such as discussed above, e.g.,SPECTRALON, covering the non-active areas of the interior cavity wall326 (much like coating 26 a in FIG. 1 above), also reflects photons 318striking the areas between the PV cells 316. This photon recyclingmechanism leads to a higher photon utilization factor and consequentlyto higher efficiencies not obtainable with flat PV counterparts.

-   -   2. Series Resistance:

At flux densities of about 50 to 100 W/cm², the series resistance ofphotovoltaic cells becomes the predominant loss mechanism and drivesdown the conversion efficiency. There are several components thatcontribute to the overall series resistance. For the sake of brevity, wemention here only three key components that are relevant in thiscomparison. The three key components are the series resistances of: (1)the metal grid path, (2) the bus-bar path, and (3) the emitter path. Thegrid resistance and bus-bar resistance are linearly dependent on thewidth and thickness of the respective metallization. Cells with widergrid- and bus bar units have lower series resistance. Emitter resistanceis proportional to the distance between the fingers. The closer thefingers are, the lower is the emitter resistance. Thus, by making thefingers and bus-bar wider and the distance between the fingers smaller,the series resistance can be reduced. However, the photocells designedfor flat panel PV receivers for concentrator systems are limited torelatively small finger widths and bus-bar widths and large distancesbetween the fingers. This is because of the shading factor F=W/d, whereW is the average finger and bus-bar width and d is the distance betweenthe fingers. As seen from this definition, a wider metallization and asmaller finger distance increases the shading factor as a result ofincreased reflective losses.

An advantage of the PowerSphere 310 is that this effect is mitigatedbecause of the photon recycling process explained above. Thus, thephotovoltaic cells 316 for the PowerSphere 310 can have a much widermetallization width and closer fingers. This capability pushes thedownturn of conversion efficiency towards much higher flux levelswithout the penalty of shadowing losses.

-   -   3. Flux Uniformity:

Flux uniformity across a high flux PV receiver is of outmost importance.A non-uniform flux that is impinging on a string of cells that areconnected in series may force a cell into reverse bias if this cellreceives less light than the neighboring cells in the string. The biasreversal occurs when the current in the string exceeds the short circuitcurrent of that cell in question. This reverse bias condition increasesthe resistive power dissipation in the cell and causes the temperatureto rise, thus forming a “hot spot”. At high flux levels, such a hot spotvery likely destroys the converter as a whole. The laser power beamprofile at a flat plate receiver is naturally not uniform across thesurface of the cell array. The flux density is high in the center anddeclines rapidly towards the edges. This is a major concern for flatplate PV receivers. On the other hand, the PowerSphere 310 of thepresent invention is equipped with a dielectric light injector 336 (asshown in FIG. 3), which eliminates this flux non-uniformity problem,since the extractor 336 of the dielectric light injector 320 distributesthe light evenly in all directions as the light 318 travels towards theits tip. The result is that in the PowerSphere 310, the flux densityacross the cells 316 is substantially uniform for the entire cavity 314.

-   -   4. Operation in Pulse Mode:

The duty cycle of some lasers (with short pulse duration) may be muchshorter than the carrier lifetime in the particular PV cells used.Although an average power output will be realized from the array as awhole, the cells must have a metal grid system that can handle the peakphoto-current to minimize the resistive losses that are proportional tothe peak current squared. The problem in the case of flat plate PVreceivers is that high metallization coverage leads to excess shadowinglosses. As discussed above, the PowerSphere 310 allows the use of highlyenhanced metallization with minimal grid coverage losses. Hence, aPowerSphere system is more suitable to operate in pulse mode than a flatplate PV receiver.

-   -   5. Additional Features of the PowerSphere for Space        Applications:

A key advantage of PowerSphere design for space applications is that thePV cells 316 are located inside the cavity 314 and are not exposed tothe space environment. Assume a sphere 310 having a housing 312 made ofberyllium or lightweight carbon composite with a thin exterior metalcladding. Such a conductive spherical structure in space provides highlyimproved space hardening for the PV cells 316. These hardening featuresprotect the cells 316 against: (a) radiation damage by charged particles(particularly for Van Allen Belt-crossing missions), (b) space chargingand power losses (Faraday Cage effect provides electro-staticallyshielding of the interior), (c) atomic oxygen, (d) space debris andmeteor showers, (e) UV, (f) solar flares and magnetic storms, etc. As aresult of these effective space-hardening features, some of theredundant, oversized beginning of life (BOL) array can be eliminated andthe specific power [$/Watt] can be improved.

For terrestrial applications, the PowerSphere 310 offers extremerobustness against damaging environmental conditions, including sandstorms, hail, acid rain, and salt spray.

The PowerSphere 310 of the present invention has the potential to yieldlaser-to-electricity conversion efficiencies from 60% to 70%. Thus,excluding any atmospheric losses, a finely tuned laser/PowerSpheresystem has the near-term potential to reach anelectricity-to-electricity conversion efficiency in the order 24 to 2%,assuming that the electricity-to-laser energy conversion efficiency is40%. If the primary energy source is a 50% efficient solar PVCCconcentrator, such as disclosed and claimed in the parent patent (U.S.Pat. No. ______), then the overall solar-to-electricity conversionefficiency via solar-PVCC/laser-PowerSphere systems becomes 12% to 14%.

INDUSTRIAL APPLICABILITY

The concentrating photovoltaic module is expected to find increasing usein space and terrestrial-based photovoltaic power systems for convertinglaser radiation to electricity.

Thus, there has been disclosed a photovoltaic module for convertinglaser light to electrical power. It will be readily apparent to thoseskilled in this art that various changes and modifications of an obviousnature may be made, and all such changes and modifications areconsidered to fall within the scope of the present invention, as definedby the appended claims.

1. A photovoltaic cavity converter module for admitting thereinconcentrated radiation produced by a laser emitting a highly collimatedpower beam of coherent light having a selected wavelength and energy,and converting the admitted laser radiation at a high efficiency ofabout 60 to about 70% into electrical power, said module comprising: (a)a housing having a cavity of generally optimized closed shape insidesaid housing, said cavity having a light input aperture or opening of aselected diameter with a total aperture area of A_(i) for the cavity,said aperture for admitting incident radiation thereon produced by thelaser into the cavity, the cavity having a total internal surface areaA_(s) and wherein the total aperture area of the opening being in aratio of up to about 0.01 of the total internal surface area of thecavity, such that the aperture allows only a relatively small portion ofthe radiation admitted into the cavity to escape out of the cavity,thereby trapping in the cavity the incident radiation admitted thereinin an amount proportional to the ratio of the total internal surfacearea to the total entrance aperture area to thereby define the totalenergy trapped in the cavity, the beam produced by the laser having adiameter greater than the aperture area; (b) a concentrator exterior ofthe housing for intercepting and concentrating the laser radiation to aselected beam diameter smaller than the diameter of the aperture and fordirecting the radiation into the housing through the light inputaperture to thereby capture the energy of the collimated laser beam; and(c) a plurality of photovoltaic cells within said cavity, saidphotovoltaic cells having an appropriate energy bandgap maximallyresponsive to said wavelength for generating said electrical power. 2.The photovoltaic module of claim 1 wherein each said photovoltaic cellis a single junction cell having a receiving surface on which said laserradiation is incident.
 3. The photovoltaic module of claim 2 whereineach photovoltaic cell is provided with a back surface mirror forreflecting photons not absorbed by a photovoltaic cell on which saidphotons are incident.
 4. The photovoltaic module of claim 2 wherein saidphotovoltaic cells have a given quantum efficiency selected to optimizethe conversion of said wavelength of said laser.
 5. The photovoltaicmodule of claim 1 wherein the concentrator comprises a primaryconcentrator and a secondary concentrator, the primary concentratorfurther including a Cassegranian comprising a parabolic concentrator forprefocusing the laser radiation, and a hyperbolic concentrator forreceiving pre-focused laser radiation from the parabolic concentratorand directing the beam into said opening.
 6. The photovoltaic module ofclaim 5 wherein said secondary concentrator has a mirrored innersurface.
 7. The photovoltaic module of claim 6 wherein said secondaryconcentrator is a non-imaging, compound parabolic of hollow design. 8.The photovoltaic module of claim 6 wherein said secondary concentratorhas a Bezier optimized contour to provide a combination of maximumacceptance angle at optimal concentration, and minimum height.
 9. Thephotovoltaic module of claim 5 wherein said secondary concentrator isdielectric and further includes an integral extractor rod for guidingsaid light towards the center of said cavity and then to emit photonsnear uniformly in all directions to provide good angular isotropy ofsaid photons.
 10. The photovoltaic module of claim 1 wherein the energyof the beam in coherent form enters the sphere where said energyscatters such that the probability of escape through the aperture isreduced in accordance with the ratio, and wherein the overallconcentration of the module is at least
 20. 11. The photovoltaic moduleof claim 1 wherein said photovoltaic cells have an optimized energybandgap to respond to said wavelength.
 12. The photovoltaic module ofclaim 11 wherein said photovoltaic cells have a peak of quantumefficiency response matching said wavelength.
 13. The photovoltaicmodule of claim 1 wherein (a) at least some of the plurality of photovoltaic cells within said cavity, have different energy bandgaps so thattheir spectral responses span different wavelength ranges; and (b) atleast one wavelength filter associated with each photo voltaic cell,said wavelength filter comprising at least one of a Rugate filter andstack interference filters, providing selective transmission andreflection of incident radiation.
 14. The photovoltaic module of claim13 wherein said photo voltaic cells are multi-junction cells.
 15. Incombination, a photovoltaic module and a concentrator system external ofthe photovoltaic module for admitting therein coherent radiationproduced by a laser emitting coherent light at a selected wavelength andinitial diameter, and converting the admitted radiation into electricalpower, wherein: (a) said module comprises: (1) a housing having a cavityof generally optimized closed shape inside said housing, said cavityhaving a total internal surface area A_(s) and including an openinghaving a selected diameter smaller than the diameter of the light, andhaving a total aperture area A_(i) for admitting said light into saidcavity, said opening having an entrance aperture area A_(i) of in aratio of up to about 0.01 of the total internal surface area of thecavity, such that the aperture allows only a relatively small portion ofthe light admitted into the cavity to escape out of the cavity, therebytrapping in the cavity the light radiation admitted therein in an amountproportional to the ratio of the total internals surface area to theentrance aperture area, the initial diameter of the light being greaterthan the diameter of the opening; (2) a plurality of photovoltaic cellswithin said cavity, said photovoltaic cells having selected appropriatebandgap energy responsive to said wavelength to generate said electricalpower; (b) said reflecting concentrator comprises: (1) a primaryconcentrator for intercepting and concentrating said light from theselected diameter to a diameter smaller that the initial diameter, and(2) a secondary concentrator coupled to the for receiving saidconcentrated light from the primary concentrator and furtherconcentrating said light from said primary concentrator to a diameterless than the diameter of the aperture and injecting the light into thehousing through the aperture; and.
 16. The combination of claim 15wherein said concentrator comprises a reflecting Cassegranianconcentrator.
 17. The combination of claim 16 wherein said Cassegranianconcentrator comprises a parabolic concentrator and a hyperbolicconcentrator.
 18. The combination of claim 15 wherein each saidphotovoltaic cell is a single junction cell having a receiving surfaceon which said laser radiation is incident.
 19. The combination of claim18 wherein each photovoltaic cell is provided with a back surface mirrorfor reflecting photons not absorbed by a photovoltaic cell on which saidphotons are incident.
 20. The combination of claim 18 wherein saidphotovoltaic cells have a given quantum efficiency selected to optimizethe conversion of said wavelength of said laser.
 21. The combination ofclaim 15 wherein the energy of the beam in coherent form enters thecavity where said energy scatters such that the probability of escapethrough the aperture is reduced in accordance with the ratio, andwherein the overall concentration of the combination is at least
 20. 22.The combination of claim 15 wherein said secondary concentrator includesinner surfaces that are mirrored.
 23. The combination of claim 22wherein said secondary concentrator is a non-imaging, compound parabolicof hollow design.
 24. The combination of claim 22 wherein said secondaryconcentrator has a Bezier optimized contour to provide a combination ofmaximal acceptance angle, maximal concentration, and minimal height. 25.The combination of claim 21 wherein said secondary concentrator isdielectric and further includes an integral extractor rod extending intothe housing for guiding said light towards the center of said cavity andto emit photons near uniformly in all directions to provide good angularisotropy of said photons.
 26. (canceled)
 27. The combination of claim 15wherein said photovoltaic cells have an optimized energy bandgap torespond to said wavelength.
 28. The combination of claim 27 wherein saidphotovoltaic cells have a peak of quantum efficiency response matchingsaid wavelength.
 29. The combination of claim 15 further including meansfor transferring waste heat from said photovoltaic module to a backsurface of said primary concentrator for radiation into the surroundingenvironment.
 30. The combination of claim 15 further including (a) aplurality of photo voltaic cells within said cavity, at least some ofsaid cells each having different energy bandgaps so that their spectralresponses span a least a portion of the spectrum of the incidentradiation; and (b) at least one wavelength filter associated with eachcell, said at least one wavelength filter comprising Rugate filters anda combination of Rugate filters and stack interference filters, therebyproviding selective transmission or reflection of incident radiation.31. (canceled)
 32. (canceled)
 33. (canceled)
 34. A photovoltaic cavityconverter module for admitting therein radiation produced by a laseremitting a power beam in the form highly collimated coherent lighthaving a selected energy, wavelength and relatively large diameter, andconverting the admitted radiation at a high efficiency of at least 60%into electrical power, said module comprising: a housing having a cavityof generally optimized closed shape inside said housing and having atotal internal surface area A_(s), said cavity having a light inputaperture of a selected diameter smaller than the relatively largediameter of the beam for admitting said beam, the aperture having atotal aperture area of A_(i) for the cavity; a concentrator external ofthe housing for intercepting the laser and reducing the diameter of thebeam to a diameter smaller than the aperture for concentrating the laserradiation and for directing the energy of the light contained in therelatively large diameter power beam produced by the laser into thesmall diameter of the aperture; the total aperture area of the aperturebeing in a ratio of up to about 0.01 of the total internal surface areaof the cavity, such that the aperture allows only a relatively smallportion of the light admitted into the cavity to escape out of thecavity, thereby trapping in the cavity the light admitted therein in anamount proportional to the ratio of the total internal surface area tothe total entrance aperture area to thereby define the total energytrapped in the cavity; and a plurality of photovoltaic cells within saidcavity, said photovoltaic cells having an appropriate energy bandgapmaximally responsive to the wavelength of the light for generating saidelectrical power; and wherein the cavity converter module exhibits anoverall concentration of at least
 20. 35. The photovoltaic cavityconverter module of claim 34 wherein the concentrator comprises a firstconcentrator stage for intercepting and reducing the diameter of thebeam to a first smaller diameter, and a second concentrator stagecoupled to the aperture for intercepting the beam of smaller diameterand further reducing the diameter to less than the diameter of theaperture and for injecting the light into the housing.
 36. Thephotovoltaic cavity converter module of claim 34 wherein the secondconcentrator stage further includes an extractor located within thecavity for distributing the light inside the cavity such that the energyof the beam in coherent form enters the cavity where said energyscatters such that the probability of escape through the aperture isreduced in accordance with the ratio.
 37. The photovoltaic cavityconverter module of claim 35 wherein the extractor comprises a rodextending from the aperture to the center of the housing for guiding thelight towards the center of the cavity and for emitting the lightsubstantially uniformly in all directions within the cavity.